MXPA04011409A - Method for producing polyesterpolyols of polyvalent alcohols. - Google Patents

Abstract

The invention relates to a method for producing polyesterpolyols of polyvalent alcohols by the mono- or polyesterification of at least one carboxylic acid containing at least two acid groups and/or at least one derivative of a dicarboxylic acid with polyvalent alcohols, optionally with the addition of a catalyst and by separating the reaction water. Said method is characterised in that the polyvalent alcohol used has a formaldehyde acetal content of less than 500 ppm.

Description

PREPARATION OF POLYESTER POLYOLS FROM POLYHYDRICAL ALCOHOLS The present. invention discloses a process for preparing polyhydric polyols of polyhydric alcohols by mono- or polyesterification of a dicarboxylic acid and / or an anhydride of a dicarboxylic acid with the corresponding polyhydric alcohols, as an option, in a solvent and, as an option, with the addition of an acid catalyst, at the same time removing the water from the reaction.

The polyhydric alcohols are those compounds having more than one hydroxyl group, for example, from 2 to 6, preferably from 2 to 4, preferably 2 or 3. These polyester polyols serve as precursors for polyurethanes, polyester resins and polyacrylates, and in this way they are used in various applications.

These applications require in particular products that have very weak colors, if they have any, without inherent smell and that have high stability in storage.

The preparation of the polyester polyols from polyhydric alcohols and dicarboxylic acids with the exclusion of water is generally known.

The reactions usually take place with catalysts, for example with an acid, or only by increasing the temperature, with or without vacuum treatment.

Since the polyester polyols of the polyhydric alcohols usually can not be purified by distillation due to their high boiling points, the by-products remain in the target ester and influence the subsequent processing and / or the quality of the target ester and also of the subsequent products.

An object of the present invention is to provide an economical process that facilitates the preparation of polyester polyols of polyhydric alcohols on an industrial scale in high purity and in high yield in a simple form and without other auxiliaries.

We have found that this objective is achieved by a process for preparing polyester polyols by reacting polyhydric alcohols with at least one dicarboxylic acid and / or a derivative, for example an ahydride, of a dicarboxylic acid, as an option in a solvent, ( at the same time removing water from the reaction, wherein the polyhydric alcohol used has a formaldehyde acetal content of less than 50 ppm.

The novel process has the following definite advantage: The final product is much less strongly colored, there are no variations in the color index between different production campaigns and fogging or condensation of the vapors of the ingredients in the product can be avoided.

The polyhydric alcohols used can be, for example, trimethylolbutane, trimethylolpropane, trimethylolethane, neopentyl glycol, pentaerythritol, 2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, glycerol, ditrimethylolpropane, dipentaerythritol, bisphenol A, bisphenol F, bisphenol B, bisphenol S, 2,2-bis (4-hydroxycyclohexyl) propane, 1 / 1-, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, 1,2-, 1,3 - or 1, 4-diclohexanediol, sorbitol, mannitol, diglycerol, erythritol or xylitol.

Preference is given to the use of these polyhydric alcohols in the process according to the invention which are obtained by reacting an aldehyde with formaldehyde and subsequently converting the aldehyde group to a hydroxyl group.

These include, for example, the polyhydric alcohols of the formula (I): where 1 2 R, R each independently are hydrogen, Ci-Cio alkyl, Ci-Cio hydroxyalkyl, C 1 -C 4 carboxyl or alkoxycarbonyl, preferably hydrogen, hydroxymethyl or C 1 -C 10 alkyl, and more preferably hydroxymethyl or alkyl of C1-C10.

The alkyl radicals can each be straight or branched chain. 1 2 The compounds of R and R include hydrogen, methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, h-hexyl, n-heptyl, n- octyl, n-decyl, hydroxymethyl, carboxyl, methoxycarbonyl, ethoxycarbonyl or n-butoxycarbonyl, and preference is given to hydrogen, hydroxymethyl, methyl and ethyl, particular preference is given to hydroxymethyl, methyl and ethyl.

Examples of the polyhydric alcohols of the formula (I) include trimethylolbutane, trimethylolpropane, trimethylolethane, neopentyl glycol, pentaerythritol, 2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, 1,3-propanediol, dimethylolpropionic acid, methyl dimethylolpropionate, ethyl dimethylolpropionate, dimethylolbutyric acid, methyl dimethylolbutyrate or ethyl dimethylolbutyrate, and preference is given to neopentyl glycol, trimethylolpropane, pentaerythritol and dimethylolpropionic acid, particularly preferred to neopentyl glycol, trimethylolpropane and pentaerythritol, very particular preference to trimethylolpropane and pentaerythritol, and in particular to trimethylolpropane.

These polyhydric alcohols of the formula (I) can be obtained, for example, by reacting an aldehyde of the formula (II): ~ 2 where R and R are each as defined above with formaldehyde and subsequently converting the aldehyde group to a hydroxyl group.

To prepare the polyester polyols according to the invention, the acids used can be carboxylic acids having at least two acid groups, preferably aliphatic or aromatic dicarboxylic acids, in particular those having from 2 to 12 carbon atoms. Examples of useful dicarboxylic acids include: adipic acid, succinic acid, glutaric acid, suberic acid, azelaic acid, sebacic acid, decandicarboxylic acid, maleic acid, fumaric acid, dimeric and / or trimeric fatty acids, and preferably adipic acid , phthalic acid, isophthalic acid, terephthalic acid and the isomeric naphthalenedicarboxylic acids. The dicarboxylic acids can be used individually or as a mixture with one another. Instead of the free dicarboxylic acids, the corresponding dicarboxylic acid derivatives, for example the dicarboxylic esters of alcohols having from 1 to 4 carbon atoms or dicarboxylic anhydrides can also be used. Preference is given to the use of mixtures of dicarboxylic acids of succinic, glutaric and adipic acid in proportions of, for example, from 20 to 35: from 35 to 50: from 20 to 35 parts by weight, and adipic acid, and in particular mixtures of phthalic acid and / or italic anhydride and adipic acid, mixtures of phthalic acid / anhydride, isophthalic acid and adipic acid or mixtures of dicarboxylic acids of succinic, glutaric and adipic acid, and mixtures of terephthalic acid and adipic acid, or mixtures of acids dicarboxylic acids of succinic, glutaric and adipic acid. For use in rigid polyurethane foams, preference is given to the use of aromatic carboxylic acids or mixtures containing aromatic carboxylic acids. In addition, preference is given to the concomitant use of fatty acids and their derivatives, and also dimeric and / or trimeric fatty acids, individually or in a mixture. Polyester alcohols based on long chain carboxylic acids, in particular fatty acids, can preferably be used to prepare alkyd resins which can also be processed for coatings.

The alcohols of the general formula (I) can be used in a mixture with other polyhydric alcohols, preferably diols, having from 2 to 12 carbon atoms, preferably from 2 to 6 carbon atoms. Examples of the di- and polyhydric alcohols, in particular diols, include: ethanediol, diethylene glycol, 1 / 2- and 1,3-propanediol, dipropylene glycol, 1, -butanediol, 1,5-pentanediol, 1, 6 hexanediol, 1, 10-decanediol, glycerol and trimethylolpropane. Preference is given to the use of ethanediol, diethylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol or mixtures of at least two of said diols, in particular mixtures of 1,4-butanediol, , 5-pentanediol and 1,6-hexanediol. It is also possible to use polyester polyols of lactones, for example e-caprolactone or hydroxycarboxylic acids, for example γ-hydroxycaproic acid and hydroxybenzoic acids.

If desired it is possible to use alcohols and / or monofunctional carboxylic acids in a mixture with the alcohols and polyfunctional carboxylic acids to adjust the functionality. Examples of the monofunctional carboxylic acids include the monomeric fatty acids, for example oleic acid or ricinoleic acid. Examples of the monomeric alcohols include aliphatic alcohols having from 1 to 15, preferably from 2 to 10 carbon atoms, for example hexanol, octanol, nonanol or decanol.

Preference is given to the use of alcohols of the general formula (I), in particular when they have a functionality greater than 2, in a maximum amount of 50% by weight, based on the weight of the polyester alcohol, since on the other hand undesired cross-linking occurs, accompanied by a high viscosity of the polyester alcohols.

To prepare the polyester polyols, the organic carboxylic acids, for example aliphatic and preferably aromatic, and mixtures of aromatic and aliphatic polycarboxylic acids and / or polyhydric alcohols and derivatives can be polycondensed in the absence of a catalyst or, preferably, in the presence of catalysts for the esterification, conveniently in an inert gas atmosphere, for example nitrogen, carbon monoxide, helium, argon, etc., in the molten state at temperatures from 150 to 250 ° C, preferably from 180 to 220 ° C, as an option under reduced pressure, up to the desired acid number which is advantageously less than 10, preferably less than 2. In order to prepare the polyether polyols, "the polycarboxylic acids and / or organic polycarboxylic alcohols and polyhydric alcohols are advantageously polycondensed in a molar ratio from 1: 1 to 1.8, preferably from 1: 1.05 to 1.2.

The catalysts used may be acid catalysts such as toluenesulfonic acids, preferably organometallic compounds, in particular those based on titanium or tin, such as titanium tetrabutoxide or tin (II) octanoate.

It is also possible to react the polyhydric alcohol initially with alkylene oxides to obtain a polyether alcohol and then esterify it with carboxylic acids. In the same way it is possible to add a catalyst for the alkoxylation to the polystyrene alcohol prepared according to the process of the invention to add an alkylene oxide. The polyether sterols obtained in this way are remarkable for their good compatibility with polyetherols and polyesterols, and can preferably be used to prepare polyurethanes.

The polyhydric alcohols (I) are obtained on an industrial scale by condensation of formaldehyde with aldehydes CH-higher acids (II) or with water and acrolein or 2-alkylacroleins. In this reaction, a difference is made between two main variants to carry out the conversion of the aldehyde group to a hydroxyl group which are shown below by the preparation of trimethylolpropane, but are by no means limited thereto.

First, there is the Cannizzaro process, which in turn is divided into inorganic Cannizzaro and organic processes. In the inorganic variant, an excess of formaldehyde reacts with the appropriate aldehyde (II), that is, n-butyraldehyde, in the presence of stoichiometric amounts of an inorganic base such as NaOH or Ca (OH) 2. The dimethylolbutanal formed in the first stage reacts in the second stage with the excess of formaldehyde in a disproportionation reaction to obtain trimethylolpropane and the format of the base used, ie, sodium format or calcium format. The presence of these salts is a disadvantage, since it is difficult to eliminate them from the product of the reaction and in addition one equivalent of formaldehyde is lost.

In the organic process of Ca'nnizzaro, a tertiary alkylamine is used instead of an inorganic base. This allows obtaining higher yields in comparison when an inorganic base is used. The trialkylammonium format is obtained as an unwanted by-product. Therefore, in the same way an equivalent of formaldehyde is lost.

The disadvantages of the Cannizzaro process are avoided by the hydrogenation process. This includes the reaction of formaldehyde with the appropriate aldehyde (II) in the presence of catalytic amounts of an amine. This achieves the interruption of the reaction considerably in the stage of the alkylated aldehyde. After removing the formaldehyde, the reaction mixture which, as well as the alkylated aldehyde mentioned, still contains small amounts of the suitable polyhydric alcohol and acetals of the alcohols formed, is subjected to a catalytic hydrogenation to obtain the desired polyhydric alcohol. Particular effective process for preparing polyhydric alcohols obtainable by condensation of aldehydes with formaldehyde is described in WO 98/28253. Through this process, high combined yields are facilitated with the presence of only small quantities of the coupling products. The process is to react the upper aldehyde with the amount from twice up to 8 times that of formaldehyde in the presence of a tertiary amine and to separate the reaction mixture obtained in this way in two solutions, one of which contains a methylolated methallylated alkanal, and the other the initial product without conversion. This latter solution is recirculated towards the reaction. The separation is effected by distillation or simple removal of the aqueous phase from the organic. The solution containing the product is subjected to a catalytic and / or thermal treatment to convert the alkylated alkanals incompletely to the fully methylolated, desired compounds. Any byproduct that is formed is removed by distillation, and the liquid phase obtained in this way is subjected to catalytic hydrogenation leading to the polyhydric alcohols.

In the process according to the invention for preparing polyester polyols, particular preference is given to the use of polyhydric alcohols of the formula (I) which have been obtained by the hydrogenation process, that is, by the reaction of an aldehyde. , of the formula (II) with formaldehyde and the subsequent conversion of the aldehyde group to a hydroxyl group by catalytic hydrogenation, more preferably those that have been obtained by the process described in O 98/28253.

For the invention it is important that the acetal content of formaldehyde in the polyhydric alcohol used be less than 500 ppm by weight, and preferably less than 400 ppm by weight.

Formaldehyde acetals (formal) are those cyclic or aliphatic compounds that contain the structural element: -O-CH2-O- (formula III) These may be complete semiacetals or acetals which are obtained from major components and impurities, or even from by-products, intermediates and the subsequent products of the reaction mixture.

These may be, for example, the following formaldehyde acetals of the "formula (IV): where R and R are each as already defined, and also R is straight-chain or branched Ci-Cio alkyl, preferably C ^ -CQ and more preferably C-Cs, Ci-Cio straight or branched chain idioxyalkyl, preferably Ci-Ce and with higher preference of I ~ CQ, O hydrogen, and n is an integer from 1 to 4, preferably from 1 to 3, and more preferably 1 or 2.

Examples of R include: hydrogen, methyl, ethyl, n-propyl, n-butyl, 2-methylpropyl, 2-methylbutyl, 2-ethyl-3-hydroxypropyl, 2-methyl-3-hydroxypropyl, 2,2-bis ( hydroxymethyl) butyl, 2,2-bis (hydroxymethyl) propyl, 2,2-dimethyl-3-hydroxypropyl, 3-hydroxypropyl, 3-hydroxy-2- (hydroxymethyl) propyl or 3-hydroxy-2,2-bis (hydroxymethyl) propyl.

The formaldehyde acetals are preferably the following: (IVa) (IVb) (IVc) 1 2 where R, R and n each are as defined.

Preferred formaldehyde acids are IVa, IVb (n-1), IVb (n = 2) and IVc.

The methanol acetals are formed from methanol which is usually present in formaldehyde at a low level, or is formed in small amounts during the preparation by a Cannizzaro reaction of formaldehyde.

In the case of the synthesis of the trihydric alcohol trimethylolpropane (TMP) from formaldehyde and n-butyraldehyde in the presence of catalytic amounts of trialkylamine, for example, the common formaldehyde acetals are IVa, IVb (n "=" 1), IVb (n = -2) and IVc, where 1 2 each R is ethyl, and each R is hydroxymethyl, each of which may be present in the crude product of the hydrogenation process in amounts from 0.05 to 10% by weight.

The content of the formaldehyde acetal is calculated from the sum of the molar weight ratio multiplied by its weight fraction determined analytically in the reaction mixture.

For example, the acetal content of formaldehyde for 1 2 a mixture of trimethylolpropane (R = ethyl, R = hydroxymethyl) containing the components (IVa), (IVb, where n = 1 and n = 2) and also (IVc), for example, it is calculated as follows: acetal content of formaldehyde [% by weight] = by weight of (IVa) x 30 g / mol +% by weight of (IVb, 146 g / mol 30 g / mol or J "/ -r-srw _" 2 x 30 g / mol 178 g / mol + ¾ T Pe TOTAL OF (IV 'n - 2) X 208 g / mol weight of (IVc) x To obtain the content of the corresponding formaldehyde acetal in ppm by weight, this value must be multiplied by 10,000.

The content of each component < it can be determined by those skilled in the art by the analytical methods known per se, for example by gas chromatography or HPLC. For example, it is possible to identify each component by coupling the aforementioned analytical methods with mass spectrometry.

It is not important for the invention how a low formaldehyde acetal content is obtained. in polyhydric alcohol.

US 6 096 905 describes a process by which a composition containing formaldehyde acetals is treated with a strongly acidic catalyst at a temperature of from 30 to 300 ° C for ½ to 8 hours.

GB-A 1 290 036 describes a process by which a crude TMP solution obtained by the inorganic Cannizzaro process is treated with a cation exchanger.

A preferred process by which the acetal content of formaldehyde can be reduced in a polyhydric alcohol consists in purifying the polyhydric alcohol after its preparation by distillation, then subjecting it to heat treatment and then purifying again, preferably by distillation, as described in the German Application with the reference number 100 29 055.8 and the application dated June 13, 2000 of BASF AG or in the international application entitled "Elimination of formaldehyde acetals from polyhydric alcohols by thermal treatment" from BASF AG.

When polyhydric alcohols are used in such a heat treatment step, particularly good results can be obtained when using alcohol solutions having a content of more than 60%, preferably > 75%, more preferred > 90%, even more preferred > 95% and in particular > 98 Examples of other components of the alcohol solutions may include solvents, for example water, methanol, ethanol or n-butanol, and also by-products that are present in the preparation of the polyhydric alcohol, preferably in amounts of less than 10% by weight, more preferably in amounts less than 5% by weight and more preferably less than 2% by weight.

This process can be used to reduce the acetal content of formaldehyde in polyhydric alcohols, preferably those alcohols of the formula (I) and in particular trimethylolpropane of any origin Charges resulting from the organic or inorganic Cannizzaro process can be used. The best results were obtained when they were used in the process alcohols that came from the hydrogenation process and that served to reduce the formaldehyde acetal In any case, it is important that the alcohol has been previously purified and have a purity in the aforementioned range .

When the process is to be used to remove formaldehyde acetals from crude solutions of polyhydric alcohols, in particular trimethylolpropane, having product contents of from 60 to 95% by weight, preference is given to subjecting the crude product obtained after the process from hydrogenation (the hydrogenation effluent) before the heat treatment step, to dehydration in which water and other low-boiling components such as methanol and trialkylamine or trialkylammonium format are removed by distillation.

To achieve the desired reduction in formaldeKido acetal content in this process, certain reaction conditions have to be maintained which may vary depending, for example, on the type of polyhydric alcohol used, the purity of the products used, the apparatus used and any another component or additives present. These reaction conditions can be obtained by those skilled in the art by experiments.

In general, the heat treatment step is carried out at temperatures from 100 to 300 ° C, preferably from 160 to 240 ° C, in times of stay from 5 minutes to 24 hours, preferably from 15 minutes to 4 hours and pressures from 100 mbar to 200 mbar, preferably from 1 to 10 bar.

When the polyhydric alcohol to be purified is trimethylolpropane, the heat treatment step is carried out at temperatures from 100 to 300 ° C, preferably from 160 to 240 ° C, times of stay from 10 minutes to 24 hours, preferably from 1 hour to 5 hours, more preferably from 30 minutes to 6 hours, and more preferably from 45 minutes to 4 hours, and to the pressures mentioned above. · To carry out the heat treatment step, those skilled in the art can use the customary apparatus continuously or in batches. During the batch operation preference is given to carrying out the heat treatment step in a vessel with stirring, and in the batch process in a tubular reactor using the liquid phase or percolate method.

The most preferred embodiment of the heat treatment step is the continuous operation in a tubular reactor in the liquid phase method.

In all these variants of operation, the reaction vessel can be provided with the customary dense packages known to those skilled in the art, for example Raschig or Pall rings, or with structured packing, for example sheet metal packing to obtain the best mixed of the components. Supports and / or catalysts may also be present in customary forms, for example extrudates or tablets, to accelerate the reactions proceeding in the heat treatment step. Examples of suitable supports / catalysts include T1O2, AI2O3, S1O2, phosphoric acid (H3PO4) with support, and zeolites.

In a variant of the heat treatment step, a suitable additive is added to the reaction solution during the heat treatment step to accelerate and facilitate the reactions that give rise to the reduction in the amounts of the formaldehyde acetals. Examples of these include not very strong and / or reducing acids or their anhydrides or ion exchangers, as described in US 6 096 905 or GB 1 290 036. Examples of suitable acids include phosphoric acid, phosphorous acid, hypophosphorous acid, boric acid, carbonic acid and sulfurous acid. Also suitable are gases, for example CO2 and SO2, which react in acid form in aqueous solution.

The acids to be used as additives are used in amounts of 10 ppm to 1% by weight, preferably 100 to 2000 ppm. Since the possibly added additive has to be removed from the polyhydric alcohol with reduced amounts of formaldehyde acetal after the heat treatment step, it is preferred that this additive be gaseous and consequently easy to remove from the reaction mixture by degassing. In addition, it may be advantageous to carry out the thermal treatment step to decompose the formaldehyde acetals under an inert gas, for example nitrogen, argon or helium, preferably under nitrogen.

Without wishing to adhere to a theory, it is suspected that the formaldehyde acetals are converted by the heat treatment step in the prepurified alcohol by distillation into higher boiling, non-volatile components and low-boiling components and from this way they can be removed by distillation more easily.

The polyhydric alcohol having a reduced formaldehyde acetal content can be easily removed from the higher boiling nonvolatile components formed by distillation. The heat treatment step is therefore generally followed by a distillation. Since the non-volatile components formed from the formaldehyde acetals in the heat treatment step usually differ markedly from the polyhydric alcohols with respect to their boiling behavior, these can be removed by simple distillation measures or the methods having just a small effect1 separator. Separating units that have only one distillation stage, for example falling film evaporators or thin film evaporators, are usually sufficient. Particularly when the distillation also serves for further purification of the alcohol product, more complicated separation processes or separating apparatuses may be used as an option, usually columns having more than one separation step, for example columns with random packing, columns with bubble cap trays or columns that have structured packaging.

The distillation is carried out using the customary conditions with respect to pressure and temperature known to those skilled in the art, although it will be appreciated that these also depend on the alcohol product used.

According to another embodiment, the heat treatment step can also be combined with distillation. In this embodiment, the heat treatment takes place at the bottom of the column of the distillation apparatus in which the polyhydric alcohol product is removed from the non-volatile components formed in the heat treatment and also any other impurities. When the heat treatment step and the distillation are combined in one step, it is important that the reaction conditions specified above with respect to the pressure, temperature and in particular residence time are maintained to obtain sufficient decomposition of the formaldehyde acetals. When the heat treatment and distillation steps are combined in a single step of the process, preference is given to the addition of acid.

The polyhydric alcohol obtainable by this process usually has a formaldehyde acetal content as defined above of less than 500 ppm by weight, preferably less than 400 ppm by weight.

The process by which the polyhydric alcohol has been obtained, for example by Cannizzaro or by the hydrogenation process, is not important.

The polyesterols prepared by the process according to the invention can react with, for example, polyisocyanates to obtain polyurethanes.

Unless otherwise mentioned, the ppm and percent data used in this document refer to percent by weight and ppm by weight.

Ex emplos' Preparation of polyester polyols for industrial rigid foam: Example I In an agitated, laboratory apparatus equipped with heating, a nitrogen sparger, a randomly packed column and the distillation apparatus, 204.1 g of oleic acid, 105.6 g of adipic acid, 214.1 g of phthalic anhydride and 541.2 g of TMP (formaldehyde acetal content 280 ppm, diformalin content and 0.4% methanol) were initially charged and heated to 170 ° C with stirring. After removing 60.4 g of water of reaction, the temperature was increased to 220 ° C. While continuing the removal of the distillate formed, the reaction was carried out to an acid number of less than 0.5 mg KOH / g.

EXAMPLE II and III (Comparative) According to Example I, the synthesis was repeated using TMP with an acetal content of formaldehyde »|| -» · | _. " of 620 ppm and 1400 ppm- * - Experiment Acetal content of color number of formaldehyde No. of TMP polyester polyols used [ppm by weight] [iodine color index] I 280 7.5 II 620 8.1 III 1400 8.6

Claims (3)

1. A process for preparing polyhydric polyols of polyhydric alcohols by mono-polyesterification of at least one carboxylic acid having at least two acid groups and / or at least one derivative of a dicarboxylic acid with polyhydric alcohols, as an option with the addition of a catalyst, eliminating at the same time the water of the reaction, wherein the polyhydric alcohol used has an acetal content of formaldehyde of less than 50 ppm.
2. 2. The process according to claim 1, characterized in that the polyhydric alcohol is an alcohol of the formula (I): where R and R are each independently hydrogen, Ci-Cio alkyl, Ci-Cio hydroxyalkyl, carboxyl or C 1 -C 4 alkoxycarbonyl.
3. 3. The process according to claim 1 or 2, characterized in that the polyhydric alcohol is trimethylolbutane, trimethylolpropane, trimethylolethane, neopentyl glycol, pentaerythritol, 2-ethyl-l, 3-propanediol, 2-methyl-1,3-propanediol, 1, 3-propanediol. The process in accordance with the claim. 2 or 3, characterized in that the polyhydric alcohol has been obtained by the reaction of the aldehyde of the formula (II): 1 2 where R and R are each as defined in claim 2, with formaldehyde, and then converting the aldehyde group to a hydroxyl group by catalytic hydrogenation. The process according to any of the preceding claims, characterized in that the polyhydric alcohol (I), after its preparation, is purified by distillation, then subjected to heat treatment and then purified again. The process according to claim 5, characterized in that the heat treatment is carried out at a temperature of from 100 to 300 ° C. The process according to claim 1, characterized in that the carboxylic acids having at least two acid groups used are the aliphatic or aromatic dicarboxylic acids having from 2 to 12 carbon atoms. · - -. The process according to claim 1, characterized in that the carboxylic acids used having at least two acidic groups are the oleic acids. The process according to claim 1, characterized in that the monofunctional carboxylic acids are used in a mixture with the carboxylic acids having at least two acid groups. The use of a polyhydric alcohol (I) having a formaldehyde acetal content of less than 50 ppm for esterification with at least one polyfunctional carboxylic acid.